1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, a plotter or a facsimile machine.
2. Description of Related Art
One type of image forming apparatus that has been conventionally proposed is an image forming apparatus using an electrode having plural opening portions (hereinafter referred to as xe2x80x9caperturesxe2x80x9d) disclosed in U.S. Pat. No. 3,689,935. In this image forming apparatus, a voltage is applied to the electrode while being modulated in accordance with image data to thereby control the passage of toner through the apertures and form an image on a supporter with the passed toner.
This image forming apparatus includes an insulating flat plate, a continuous reference electrode formed on one surface of the flat plate, and plural control electrodes formed on the other surface of the flat plate and electrically insulated from one another. The apparatus further includes an aperture electrode member having at least one array of apertures, each of which is provided in correspondence with each control electrode so as to penetrate through the flat plate, the reference electrode and the control electrode. Also provided are means for selectively applying potential between the reference electrode and each of the control electrodes, means of supplying charged toner to modulate the flow of toner that has been just passed through the apertures in accordance with the supplied potential and means for positioning the supporter in a toner-particle flowing passage such that the supporter and the aperture electrode member are movable relatively to each other.
However, in the above-mentioned image forming apparatus, because an electric field in the apertures is used to control the flow of the charged toner particles, an electric field outside the apertures is weakened significantly. Therefore, the amount of the toner particles passing through the apertures in a predetermined time is slow. Thus, the image forming speed is slow, since the electric force for drawing toner particles supplied under the apertures into the apertures is weak.
An object of the present invention is to provide an image forming apparatus having a stable and excellent image forming property.
To improve the strength of the electric force drawing toner particles through apertures in an aperture electrode member, the toner particle carrying roller is arranged to contact the aperture electrode member. By this arrangement, an image is formed by controlling the toner particle flow with a strong electric field formed between the control electrode of the contacting aperture electrode member and a toner particle carrying roller.
The aperture electrode member is preferably formed with an insulative sheet such as a polyimide film. However, such an insulative sheet will become charged and worn off due to its sliding operation with the toner carrying roller. Further, toner particles will become fixed on the insulative sheet, and, therefore, a proper control electric field cannot be formed between the control electrodes and the toner carrying roller since the insulative sheet is charged by the sliding operation with a toner particle layer formed on the toner carrying roller. Moreover, the life of the insulative sheet will be shortened since the insulative sheet will wear off due to the sliding operation and toner particles that become fixed on the insulative sheet. Therefore, to prevent these problems, an insulative inorganic substance is provided on at least a surface of the insulative sheet facing the carrying roller.
To achieve the above and other objects, the image forming apparatus of the present invention comprises a supporting member for supporting and supplying charged toner particles, and toner flow control means having openings and control electrodes around the openings. The toner flow control means sandwiches the charged particles with the supporting member and controls a flow of the charged particles through the openings by an electric field. A back electrode is provided that confronts the toner flow control means sandwiching an image receiving medium therebetween. Additionally, an insulative inorganic substance is provided on at least a surface of the toner flow control means facing the supporting member.
In the image forming apparatus of the above-described structure, the electrodes themselves are not charged and not worn off because the insulative inorganic substance is provided on the electric field control means. Therefore, toner particles are always supplied stably, and an unnecessary electric field is not formed when toner particles are transferred. As a result, the image forming apparatus has an excellent image forming property and excellent product stability.